[remarkEon]

If I were to be the one deciding how to measure this, the “biggest” subway would be the one that moves the most people per day.

[magicalhippo]

For me, complexity would have to be a factor.

A single ring line would in my view count as smaller than a more traditional "flared out" system, assuming they moved the same amount of passengers.

Maybe a complexity factor based on number of distinct tunnels multiplied by number of riders? Hmmm, metrics are hard...

[vmception]

Nah, breadth and depth

[peterpost2]

dont forget width!